fix(typescript): Add missing / to default types path (projen#133)

*Issue projen#132, if available:*

*Description of changes:*
This adds the missing `/` in the default `types` path in `TypeScriptProject`

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src/typescript.ts

@@ -167,7 +167,7 @@ export class TypeScriptProject extends NodeProject {
       });
     }
 
-    this.manifest.types = options.entrypointTypes ?? `${path.dirname(this.entrypoint)}${path.basename(this.entrypoint, '.js')}.d.ts`;
+    this.manifest.types = options.entrypointTypes ?? `${path.join(path.dirname(this.entrypoint), path.basename(this.entrypoint, '.js'))}.d.ts`;
 
     const compilerOptions = {
       alwaysStrict: true,